There is a very distinct mechanical clicking/whirling sound that is intermittently happening somewhere near the air intake of my intake manifold. It almost sounds like one of those sensors there.

It'll make the nosie for like 10-15 seconds, then stop for a bit, then randomly start again. It makes a very consistent rhythmic click - like a mechanical switch opening/closing very fast.

All my googles lead to exhaust leak or lifter tick. I have the lifter tick so I know its not that. I don't think its exhaust leak either as my RPMs are normal, and the car drives wonderfully. I just have very bad cabin vibes when sitting in gear with brakes on.

I took a hose and listened and am very confident it is near the TPS. Are any of those intake sensors known to have a relay or make this noise?

Possible but it doesn't sound like its coming from there. I know its hard to be confident of that though. I pulled each FI and the XJ stumbled every time so they are all working correctly. Next time I hear the tick I'll do it again and see if it stops.

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Originally Posted by ID96XJ

Could it be the IAC?

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Originally Posted by 90xj06

the iac is a stepper motor and when they get stuck they do make click like noises. but the iac is so small im not sure you could hear it.

IAC is definitely in the area I am hearing it. It is very faint and I have to put my head up to the manifold to really hear it. You would never hear it with the hood closed or from the cab.

I recently had the manifold removed and sitting the garage for ~6 months. Perhaps the IAC got dirty or gummed up? I cleaned it a while back, but didn't really bother cleaning again before throwing it back in.

I'm just tracking down a rough idle, and I know my brake booster is bad so hoping its just causing a minor vaccum leak. So I will tackle that first and if it doesn't make a difference I will troubleshoot the IAC. My only problem with that is how smooth the RPMs are.

Does the rough idle change if you plug the brake booster line? If not, that's not your vacuum leak.

I will try this out. Either way I know the booster is bad, I have a stiff pedal that won't go away. I should be more specific too - the rough idle is a low frequency cabin shake that only happens when sitting in gear (R/D) with my foot on the brakes. If I'm in P, N, or driving - it's smooth.

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Originally Posted by 90xj06

iac does control the idle speed and if its all gummed and not working that could be a problem.

When you say speed are you talking RPMs? I idle very steady at ~700RPM and it barely budges. Still worth cleaning again like you say.